(1) Field of the Invention
The invention relates to a method of manufacturing a silicon condenser microphone, and more particularly, to a method of manufacturing a high performance silicon condenser microphone using a silicon micro-machining process.
(2) Description of the Prior Art
Silicon condenser microphones have long been an attractive research and development subject. Various microphone designs have been invented and conceptualized by using silicon micro-machining technology. Despite various structural configurations and materials, the silicon condenser microphone consists of four basic elements: a movable compliant diaphragm, a rigid and fixed backplate (which together form a variable air gap capacitor), a voltage bias source, and a pre-amplifier. These four elements fundamentally determine the performance of the condenser microphone. In pursuit of high performance; i.e., high sensitivity, low bias, low noise, and wide frequency range, the key design considerations are to have a large size of diaphragm and a large air gap. The former will help increase sensitivity as well as lower electrical noise, and the later will help reduce acoustic noise of the microphone. However, the large diaphragm requires a large span of anchored supports and correspondingly a large backplate. Also, a large air gap requires a thick sacrificial layer. These present major difficulties in silicon micro-machining processes. Due to constraints of material choices and intrinsic stress issues in silicon micro-machining, the silicon microphones reported so far have not achieved sensitivity of more than 20 mV/Pa.
Miniaturized silicon microphones have been extensively developed for over sixteen years, since the first silicon piezoelectric microphone reported by Royer in 1983. In 1984, Hohm reported the first silicon electret-type microphone, made with a metallized polymer diaphragm and silicon backplate. And two years later, he reported the first silicon condenser microphone made entirely by silicon micro-machining technology. Since then a number of researchers have developed and published reports on miniaturized silicon condenser microphones of various structures and performance.
Some of these reports include the following:
1) D. Hohm and R. Gerhard-Multhaupt, xe2x80x9cSilicon-dioxide electret transducerxe2x80x9d, J. Acoust. Soc. Am., Vol. 75, 1984, pp. 1297-1298.
2) D. Hohm and G. Hess, xe2x80x9cA Subminiature condenser microphone with silicon nitride membrane and silicon backplatexe2x80x9d, J. Acoust. Soc. Am., Vol. 85, 1989, pp. 476-480.
3) Murphy, P. et al., xe2x80x9cSubminiature silicon integrated electret capacitor microphonexe2x80x9d, IEEE Trans. Electr. Ins., Vol. 24, 1989, pp. 495-498.
4) Bergqvist, J. et al., xe2x80x9cA new condenser microphone in siliconxe2x80x9d, Sensors and Actuators, Vol. A21-23, 1990, pp. 123-125.
5) Kuhnel, W. et al., xe2x80x9cA Silicon condenser microphone with structured backplate and silicon nitride membrane,xe2x80x9d Sensors and Actuators, Vol. 30, 1991, pp. 251-258.
6) Scheeper, P. R. et al., xe2x80x9cFabrication of silicon condenser microphones using single wafer technologyxe2x80x9d, Journal of Microelectromechanical Systems, Vol. 1, No. 3, 1992, pp. 147-154.
7) Scheeper, P. R. et al., xe2x80x9cA Review of Silicon Microphonesxe2x80x9d, Sensors and Actuators A, Vol. 44, July 1994, pp. 1-11.
8) Bergqvist, J. et al., xe2x80x9cA Silicon Microphone using bond and etch-back technologyxe2x80x9d, Sensors and Actuators A, vol. 45, 1994, pp. 115-124.
9) Zou, Quanbo et al., xe2x80x9cTheoretical and experimental studies of single-chip-processed miniature silicon condenser microphone with corrugated diaphragmxe2x80x9d, Sensors and Actuators A, Vol. 63, 1997, pp. 209-215.
10) Brauer, M. et al., xe2x80x9cSilicon microphone based on surface and bulk micromachiningxe2x80x9d, Journal of Micromech. Microeng., Vol. 11, 2001, pp. 319-322.
11) Bergqvist, J. and V. Rudolf, xe2x80x9cA silicon condenser microphone with a highly perforated backplatexe2x80x9d, Transducer 91, pp. 266-269.
U.S. Pat. No. 5,870,482 to Loeppert et al reveals a silicon microphone. U.S. Pat. No. 5,490,220 to Loeppert shows a condenser and microphone device. U.S. patent application Publication 2002/0067663 to Loeppert et al shows a miniature acoustic transducer. U.S. Pat. No. 6,088,463 to Rombach et al teaches a silicon condenser microphone process. U.S. Pat. No. 5,677,965 to Moret et al shows a capacitive transducer. U.S. Pat. Nos. 5,146,435 and 5,452,268 to Bernstein disclose acoustic transducers. U.S. Pat. No. 4,993,072 to Murphy reveals a shielded electret transducer.
However, none of the silicon condenser microphones mentioned above has been reported to achieve sensitivity above 20 mV/Pa. In terms of conventional condenser microphones (i.e. non-silicon), very few products can have sensitivity as high as 100 mV/Pa. For example, Bruel and Kjoer, Denmark (BandK) has only one microphone available with this high sensitivity (BandK 4179, 1-inch diameter). Its dynamic range is about 140 dB (200 Pa) and frequency range is 5-7 kHz. However, this microphone must be fit onto a bulky pre-amplifier and requires a polarization voltage of 200V.
A principal object of the present invention is to provide an effective and very manufacturable method of fabricating a silicon condenser microphone having high sensitivity and low noise.
Another object of the invention is to provide a silicon condenser microphone design having high sensitivity and low noise.
A further object of the invention is to provide a method for fabricating a silicon condenser microphone using via contact processes for a planar process.
Yet another object of the invention is to provide a method for fabricating a silicon condenser microphone using buckling of a composite diaphragm to prevent stiction in a wet release process.
In accordance with the objects of this invention a silicon condenser microphone is achieved. The silicon condenser microphone of the present invention comprises a perforated backplate comprising a portion of a single crystal silicon substrate, a support structure formed on the single crystal silicon substrate, and a floating silicon diaphragm supported at its edge by the support structure and lying parallel to the perforated backplate and separated from the perforated backplate by an air gap.
Also in accordance with the objects of this invention a method of fabricating a silicon condenser microphone having high sensitivity and low noise is achieved. A single crystal silicon substrate (Pxe2x88x92) is provided. First ions (P+) of a first conductivity type are implanted into the single crystal silicon substrate to form a pattern of acoustic holes in a central portion of the substrate. Second ions (Nxe2x88x92) of a second conductivity type opposite the first conductivity type are implanted into the substrate and surrounding the pattern of acoustic holes to form a backplate region. Third ions (P+) of the first conductivity type are implanted overlying the pattern of acoustic holes. Fourth ions (N+) of the second conductivity type are implanted overlying a portion of the backplate region not surrounding the pattern of acoustic holes to form an ohmic contact region. A front side nitride layer is deposited overlying the backplate region. A back side nitride layer is deposited on an opposite surface of the substrate. A front side sacrificial oxide layer is deposited overlying the front side nitride layer. A back side sacrificial oxide layer is deposited overlying the back side nitride layer. First trenches are etched through the front side sacrificial oxide layer to the ohmic contacts, and to the substrate off the backplate region. The first trenches are filled with a first polysilicon layer which is patterned to form polysilicon caps overlying the first trenches and to form polysilicon end plates surrounding the pattern of acoustic holes. A first oxide layer is deposited overlying the patterned first polysilicon layer. The first oxide layer is etched to the polysilicon layer followed by a thin oxide deposition to form the tiny holes for first dimples overlying the end plates. A second polysilicon layer is deposited overlying the first oxide layer and filling the first dimple holes. The second polysilicon layer is etched to form a functional layer of a composite diaphragm and its lead-out to a bond pad. A second oxide layer is deposited overlying the first oxide layer and the functional diaphragm. A narrow and continuous opening on the second oxide layer is etched on an inner edge of the functional diaphragm. A third polysilicon layer is deposited overlying the second oxide layer and filling the openings whereby a portion of the second oxide layer is enclosed between the second and third polysilicon layers to form a compressive layer of the composite diaphragm. The third polysilicon layer is patterned to remain filling the narrow and continuous opening to form a protective layer over the compressive layer of the composite diaphragm. The first and second oxide layers are etched followed by a thin oxide deposition to form second dimple holes overlying the first dimples. A deep oxide trench etching is made through the end plates and the sacrificial oxide layer to the substrate to form the supporting struts. The first and second oxide layers are etched to make anchor openings to the polysilicon caps, end plates, and bond pads. A nitride layer is deposited overlying the second oxide layer and filling the second dimple holes, the oxide trenches and the anchor openings. The nitride layer is patterned to expose the bond pads and the composite diaphragm within the second dimples. Thereafter, the backside sacrificial oxide layer is removed and the backside nitride layer is patterned. From the backside, the silicon substrate is etched away to the backplate region. The pattern of acoustic holes is selectively etched away. The backside nitride layer and the frontside nitride layer exposed by the acoustic holes are etched away from the backside. The frontside sacrificial oxide layer is removed using a wet etching method wherein the compressive layer of the composite diaphragm causes the composite diaphragm to buckle in a direction away from the backplate region. After drying, the protective layer and the compressive layer of the composite diaphragm are removed wherein the functional diaphragm flattens to complete fabrication of a silicon condenser microphone.